The Role of the Chromosomes 



217 



female stick together in the reduction division, so that 

 one class of eggs receives two X and the other none. A 

 detailed analysis of all the possible combinations resulting 

 from such a case is rather too complicated for consideration 

 here. Suffice it to say that the unexpected class of white- 

 eyed females could be obtained by the fertilization of an XX 

 egg by a Y sperm, and that of red-eyed males by the union 

 of an egg lacking an X with an X sperm (the X being the 

 important chromosome in sex determination, the Y apparently 



DIAGRAMMATIC REPRESENTATION OP THE CHROMOSOMES OF THE 



FRUIT FLY 



The female (left) and male (right). The chromosomes are sho\rn in 

 pairs, the sex pairs being indicated by XX (female) and XY (male) 

 respectively. In the lower figure is shown the peculiar case of a female 

 carrying 2 X and 1 Y, owing to the failure of the 2 X to separate from 

 each other in the ripening divisions of the egg. This condition explains 

 certain unusual results found by Morgan and his students in breeding 

 these flies for eye color. After Morgan et al. 



non-functional in this connection, 2 X giving a female and 

 1 X a male). It is also possible theoretically to obtain a 

 female with four sex chromosomes, 2 Xs and 2 Ys. Not only 

 can the unusual breeding results be explained theoretically 

 in this manner, but cytological research shows that such cases 

 actually occur, some exceptional females having been found 

 which contained one or two Y chromosomes in addition to 

 the usual XX. 



